In the packaging industry, it is conventional that a product is packaged in a plastic film or a rigid plastic package which is then heat sealed.
Conventional heat-sealing plastic films such as polyvinyl chloride, polyethylene and polypropylene films are not completely satisfactory in terms of their performance characteristics. For example, polyvinyl chloride films generate toxic pollutants when incinerated, and polyethylene films have been employed only for low-grade packaging materials due to their relatively poor dimensional and mechanical properties. Heat-adhesive polypropylene films, on the other hand, have satisfactory mechanical properties, but generate unbiodegradable decomposition products. In order to solve such problems, there have been employed biodegradable aliphatic polyesters, particularly polylactic acid films. However, such films have poor heat adhesiveness and flexibility due to their high crystallinity.
In order to solve such problems, there have been employed biodegradable aliphatic polyesters, particularly polylactic acid-based films.
For example, Japanese Laid-open Patent Publication Nos. 1997-157408 and 2004-109000 disclose a multilayered film composed of a polylactic acid polymer, and an aliphatic polyester resin having a glass transition temperature (Tg) of 0° C. or lower. However, the heat shrinkages of these films are significantly high due to their relatively poor heat resistance. Further, the seal is not easily openable by pulling with hands.
Japanese Laid-open Patent Publication No. 2003-170560 discloses a multilayered film comprising an amorphous lactic acid-based resin layer as a heat-sealant layer. However, the film is not satisfactory for packaging because the film turns cloudy while the package is subjected to a hot bath treatment.